In general, modems designed for use with conventional telephone lines accommodate relatively low data transmission rates. While current modems can process a high-end bit rate at about 33.6 Kbits/second, they are nonetheless significantly slower than a digital modem, such as one on an ISDN line which can operate at 64 Kbits/second. These rates, unfortunately, remain too low for many desired types of communication, such as full-motion video which requires a minimum of 1.5 Mbits/second for VHS quality using MPEG-1 (Motion Pictures Expert Group) compression and 3 to 6 Mbits/second for broadcast quality using MPEG-2 compression.
A recently pronounced standard in telecommunications defines an Asymmetrical Digital Subscriber Line (ADSL) system which executes a high speed transfer of data over a single twisted-wire pair, such as an existing telephone line. In addition to Plain Old Telephone Services (POTS), an ADSL system also permits full-duplex and simplex digital services with data rates from about 1.5 Mbits/second to 7 Mbits/second. An ADSL system uses a spectrum from about 26 kHz to 1.1 MHz for broadband data transmission and leaves the spectrum from about DC to 4 kHz for POTS. An ADSL system provides at least four downstream simplex channels having rates ranging from about 1.5 Mbits/second to 6 Mbits/second and four full duplex channels with rates ranging from about 64 Kbits/second to 640 Kbits/second. An ADSL system is therefore more than capable of providing video-on-demand capability, video conferencing, data file transfer capability and can provide all of this capability simultaneously with POTS. For additional information, reference may be made to American National Standards Institute Standard ANSI-T1.413-1995 which describes an ADSL system and an interface between a telecommunications network and a customer's installation and which is incorporated herein by this reference.
With reference to FIG. 1, a standard ADSL system 10 may comprise an ADSL transceiver unit 12 at a central office (ATU-C) which communicates with an ADSL transceiver unit 14 at a customer premises (ATU-R). The ADSL transceiver unit 12 at the central office receives data from a digital network 15, performs various processing on the data, and transfers the processed data to a splitter 16. The splitter 16 combines the signals from the transceiver unit 12 with signals from a public switched telephone network (PSTN) 18 and transfers the combined signals onto a line 20. At the customer end, a splitter 22 supplies a lower-band set of signals to one or more POTS terminal devices 24 and a higher-band set of signals to the ADSL transceiver unit 14. The ADSL transceiver unit 14 at the customer's end processes the received signals and supplies the processed signals to one or more service modules (SM) 26. The processed data from the ADSL transceiver unit 14 may be supplied directly to the service modules 26 or may be supplied through a customer installation distribution network 28. The network 28 may be any type of network, such as a star or bus network. Reference may be had to ANSI T1.413-1995 for additional information on the ADSL transceiver units 12 and 14 and on other aspects of the ADSL system 10.
One difficulty with ADSL, however, is that the signals supplied to the ADSL transceiver 14 and the signals supplied to the POTS terminal device 24 must be isolated from each other. One reason requiring this isolation is that the POTS terminal device 24, which may be a telephone or other non-linear device, produces inter-modulation harmonics from the ADSL system both in the frequency range of the ADSL signals and in the voice band. Likewise, the ADSL transceiver unit 14 can generate interference with the signals supplied to the POTS terminal device 24. Consequently, some type of filtering must occur between the ADSL transceiver 14 and the POTS terminal devices 24.
The signals supplied to the POTS terminal devices 24 may be isolated from the signals supplied to the ADSL transceiver unit 14 in any one of a multitude of ways. One of these ways is to place a low-pass filter at each POTS terminal device 24 and to place a high-pass filter at either the ADSL transceiver unit 14 or at a network interface device (NID). For instance, the low pass filters may be placed in series between the POTS terminal devices 24 and their connection to a wall jack. These low pass filters would then filter out the higher band ADSL signals and prevent the ADSL signals from interfering with the POTS signals.
The placement of the low-pass filter at each POTS terminal device, however, adversely effects the overall performance of the ADSL system 10. The lines connecting the POTS terminal devices 24 to the low pass filters look like bridge taps to the ADSL line and produce significant losses at the top end of the downstream ADSL band transfer function, such as losses from 5 dB to 15 dB between 400 kHz and 1.1 MHz. The reason for these losses and their effects on the ADSL system 10 are explained in more detail in Dennis J. Rauschmayer, "Effects of a Distributed POTS Splitter Topology on ADSL Line Transfer Functions," American National Standards Institute T1 E1.4 Technical Subcommittee Report T1E1.4/96-167, Jul. 22, 1996, which is incorporated herein by this reference. The placement of low-pass filters at each POTS terminal device 24 is therefore undesirable due to their effects on the ADSL signals.
In contrast to the placement of a low-pass filter at each POTS terminal device 24, the use of a single low pass filter for all POTS terminal devices 24 produces more favorable results. For instance, a comparison between the placement of the low-pass filter at each phone drop versus the placement of the low-pass filter at a split is described in a report by Rick Roberts et al., "ADSL POTS LPF Placement," American National Standards Institute Working Group Report T1E1.4/96-162, July, 1996, which is incorporated herein by this reference. This report suggests that a single low-pass filter at the split is preferred since a distributed low-pass filter at each phone causes several problems, such as a reduced bit rate and reduced reach of the ADSL system, an increase in line driver current, a hybrid/echo cancellation stress, and risk of improper installation or improper network modification. Thus, rather than placing a low-pass filter at each POTS terminal device 24, the ADSL system 10 should preferably have a single low-pass filter installed at the split so that the signals supplied to all of the POTS terminal devices 24 are filtered by this single low pass filter.
A single low-pass filter, however, is not as easily installed at a split as are multiple low-pass filters at each POTS terminal device 24. With multiple low-pass filters, a low-pass filter can be easily incorporated to the telephone network by simply adding a filter between each POTS terminal device 24 and its connection to the customer's telephony wiring, such between the POTS terminal device 24 and a wall jack. The single low-pass filter, on the other hand, must be located at a point along the customer's wiring which is shared by all POTS terminal devices 24 but not at a location which might effect ADSL signals traveling to and from the ADSL transceiver unit 14.
This difficulty in placing a single low pass filter at the split will be explained with reference to FIGS. 2A and 2B, which depict a conventional ADSL installation within a network interface device (NID). In many households and businesses, especially those more recently constructed, a telephone company's wiring is interconnected to the particular customer's telephone wiring within the NID and this interconnect is protected from the elements of the environment within the NID. The point at which the customer's wiring is connected to the telephone company's wiring is termed the customer demarcation point.
With reference to FIG. 2A, a typical NID 32 has a station protector 38 for receiving an incoming service wire 34 and a ground wire 36. Typically, the service wire 34 is attached to the station protector 38 so that a tip signal is supplied to a left post 33A on the protector 38 and a ring signal is supplied to a right post 33B on the protector 38. A pair of leads 40 from a customer bridge 42 couples the tip and ring lines from the station protector 38 to the customer bridge 42 which, in this example, is through an RJ11 female connector 44. The RJ11 female connector 44 defines the customer demarcation point and thus defines the intersection of the telephone company's wiring and that of the customer's telephone wiring. An RJ11 male connector 45 is connected to a cord 46 which couples the tip and ring signals to the binding posts 50 on the customer bridge 42. The RJ11 male connector 45 and the RJ11 female connector 44 provide a convenient testing jack whereby test equipment can be coupled to the RJ11 female connector 44 to ensure that telephony signals are properly reaching the customer's premises. An ADSL interconnect wire 52, connected to posts 50, supplies signals to and from the ADSL transceiver unit 14 and a telephony or POTS interconnect wire 54, also connected to posts 50, supplies telephony signals to and from the POTS terminal devices 24 located within the customer's premises. Although each of the service wire 34, ADSL interconnect wire 52, and POTS interconnect wire 54 has been referred to as a wire, as shown in FIG. 2A, each of these wires 34, 52, and 54 is a line comprised of a pair of conductors for carrying signals.
The customer bridge 42, as shown in FIG. 2B, is a unitary modular structure that is releasably secured to the NID 32 by snapping the bridge 42 within a receptacle 35 formed within the NID 32. The NID 32 has engaging members 37 for engaging edges of the customer bridge 42 so as to secure the bridge 42 to the NID 32. The station protector 38, service wires 34, and grounding wire 36 are each located within a central compartment of the NID 32 and this central compartment is often locked under separate cover so as to prevent any tampering with the wires 34 and 36. Consequently, once the customer bridge 42 has been installed and the leads 40 have been connected to the station protector 38, the customer bridge 42 cannot be removed while the central compartment is locked except by cutting the leads 40.
The installation shown in FIG. 2A lacks any type of filtering for either the POTS signals or the ADSL signals. If the filters for the POTS and ADSL signals are to be considered part of the customer's own telephone wiring, the low pass filter and high pass filter must be placed after the customer demarcation point, which in this example is the RJ11 female connector 44. Additionally, the low pass filter must be located before any branching occurs to separate POTS terminal devices 24 in order for the single low pass filter to provide filtering for all of the POTS terminal devices 24. The filters should also be protected from the elements of the environment to prevent moisture or dirt from damaging the filters.
The filters are not readily placed in its desired position within the customer's telephone wiring if it is to be placed in a protected enclosure and located before any branching to separate POTS terminal devices 24. One possible location for the low pass filter satisfying all of these desires is within the NID 32 itself The typical NID 32, however, is a fairly small enclosure and does not have much space for any extra components since much of the space is consumed by the customer bridge 42. The low and high pass filters therefore are not readily located within the NID 32.
Another possible location for the filters is external to the NID 32 but before the interconnect wire 54 enters the customer's premises. At such a location, the interconnect wire 54 may have to be cut in order to splice in the low pass filter, which consequently would increase the chance of a service interruption due to a faulty splice. This option is also undesirable since the splice must be encased and protected from the environment, thereby complicating the installation of the low pass filter.
A third general location for the low pass filter is within the customer's premises at a point prior to any branching to the separate POTS terminal devices 24. This option is often plagued with problems since the customer's wiring is usually completely hidden behind a wall or floor. Simply locating the wiring behind the wall or underneath the floor may be quite difficult and, even if the wiring is found, the wall or floor may have to be partially removed to access the wiring, which just presents the customer with the additional task of repairing the wall or floor. The placement of the low pass filter within the customer's premises therefore may involve the most amount of effort and is thus likely to be the least desirable location for the low pass filter.
A further difficulty in the placement of the low pass filter is that the customer or one with a minimal amount of training should preferably be capable of installing the low pass filter. Because ADSL relies upon existing telephone wires and does not require any additional digital lines, ADSL by its very nature can be easily incorporated into many households or businesses. To help minimize the number of obstacles to the provision of ADSL services, the low pass filter should be designed and located so that potential customers with little or no experience in working with electrical lines or circuits can install the low pass filter. This goal of simplifying the installation of the low pass filter may be difficult to achieve given the minimal amount of space within the NID 32 and the level of expertise needed to splice the low pass filter external to the NID 32.